The present invention relates to a process for foaming polystyrene, which has been given a flame retardant treatment, by addition of blowing agents, and to a shaped article of extruded polystyrene foam.
Combustible gases are in general employed as blowing agents for foaming in the field of building materials which have been given a flame retardant treatment, such as sheets of plastic, expanded films, thermal insulation film webs and the like. These materials are produced by extrusion from foamed melts of plastic. The combustibility of these blowing agents or blowing gases is in conflict with the requirement for flameproofing, so that such building materials develop their full flame retardant properties only after a relatively long storage period, during which the blowing gas can escape from the cells of the foamed plastic.
German Utility Model No. 92 11 584 discloses a shaped article of polystyrene recycled material which is obtained by melting scrap polystyrene foam to form polystyrene, in order to allow the air enclosed to escape, and then converting the melt into expanded polystyrene, after addition of blowing agent. Pentane is used as the blowing agent, with the foaming operation taking place in the head of an extruder. The known shaped article is produced in the form of so-called continuous sheets, which are cut into the desired lengths and widths and then further processed as building material sheets. The known shaped article contains a flameproofing additive, which renders it "flame retardant" according to building materials class B1 in accordance with DIN 4102. The known shaped article of polystyrene recycled material is used as an insulating material for the building industry and meets the requirements of DIN 18 164 Part 1.
To prepare the polystyrene foam as the starting material for the known shaped article, the melt of scrap polystyrene is cooled to a temperature below the melting temperature and pressed into the shape of filaments, and the cooled filaments are comminuted into pieces. These component pieces are melted in another extruder, pentane is added as the blowing gas, which is forced in under a high pressure, and the melt is then extruded as a flexible foam via a slot die at temperatures below 150.degree. C., where it is shaped by calibration. The storage time required before the full flame retardant properties of these building materials are achieved is not referred to.